One typical steam generator for a pressurized-water coolant nuclear reactor includes a vertical jacket having a steam output outlet in its top, the jacket's bottom being closed by a horizontal tube sheet in which the inlet and outlet ends of an inverted U-shaped tube bundle are mounted, the jacket having a feed water inlet, the water being the secondary medium, and from which steam is generated. Primary medium chambers beneath the tube sheet provide for passing the pressurized-water coolant through the tube bundle via its inlet and outlet ends. To provide high steam generating capacity, the jacket, tube bundle and its tube sheet have large diameters to a degree undesirable from the manufacturing, transportation and erection cost.
Another type of steam generator is disclosed by the journal "Nuclear Engineering", October 1957, pages 433 and 435. In this form two horizontal U-shaped tube bundles are used, positioned end-to-end, inside of a single horizontal cylindrical jacket which encloses both legs of both tube bundles. This means that the jacket must be large enough in diameter to enclose both bundle legs in each instance, and the arrangements for connecting the opposite legs involve undesirable complications.